1955 measurements

The measurements have been made after the circuit has been operated for one hour at room temperature approximately 25°C.

We have 290 V on the C9 filter capacitor instead of 310 V, maybe because our mains transformer has higher internal resistance than the one used in the Philips EZ81 graphs, which was our reference for the power supply calculations.

In my prototype I have tried using a higher voltage transformer, but the voltages of the circuit were too high, and I finally decided to use the 370DAX.

The output transformer also doesn’t have the same primary winding resistance as the one used in the original circuit. We have

Rp = (310 – 290) / 0.045 = 444 Ω

444 Ω in the original transformer versus 350 Ω measured in the 125BSE. It was necessary to change the value of the resistor R8 from 3.9 kΩ to 2.2 kΩ to equalize the plate and screen voltages of V2.

The frequency-response characteristic for an output power of 0.3 W RMS is shown in the graphs below. It will be seen that the frequency response is level (to within 2 dB) from 20 Hz to 20 kHz. Measurements corresponding to the “improved circuit” graphs have been made without the feedback capacitor C5 and with the capacitor C8 capacity increased to 100 uF.

Frequency Response and Phase Shift (standart circuit)

Frequency Response and Phase Shift (enhanced circuit)

Distortion

Distortion (stardard circuit)

Distortion (enhanced circuit)

Linearity

Relationship between input voltage and output power.

Sensitivity

The sensitivity of the amplifier measured at 1 kHz is 100 mV RMS for an output of 3.48 W RMS.

Input : 100 mVrms 1 kHz

V1a : 22.8 Vdc + 3.33 Vrms 1 kHz

V2a : 296 Vdc + 132 Vrms 1 kHz

We can calculate the output power

P = U2 / RLOAD = 1322 / 5000 = 3.48 WRMS

For an input of 100 mV, the output is 132 V. Hence the output is 132 / 0.1 = 1320 times as great as the input (62 dB).